Pre-heating lamp filaments helps extend the life of gas-discharge lamps. Furthermore, the pre-heating of lamp filaments may be necessary to ignite certain types of high impedance gas-discharge lamps, such as the T5 model gas-discharge lamp. During a pre-heat period, ballast circuits may be coupled to pre-heat components to transmit pre-heat voltages to the lamp filaments of gas-discharge lamps 16.
Referring now to FIG. 1, one example of a prior art ballast circuit 1 is shown. Half-bridge class D inverters are commonly used to operate high-impedance gas discharge lamps because of their efficiency and relatively inexpensive design. The ballast circuit 1 in FIG. 1 utilizes a half-bridge Class D inverter 3 to power two high impedance gas-discharge lamps 4, such as the T5 model gas discharge lamps. Inverter 3 converts a DC voltage from V_rail into an AC voltage that powers high impedance gas-discharge lamps 4. To convert the DC voltage into the appropriate AC voltage, the inverter 3 utilizes inverter drive circuit (not shown) and switches Q1, Q2 to convert the DC voltage into a pulsed voltage. DC blocking capacitor, C_dc_block, blocks the DC components of the pulsed voltage. In cooperation with resonant circuit 5, the pulsed voltage is converted into an AC voltage having the appropriate frequency for operating the gas discharge lamps 4.
To pre-heat the lamp filaments 4A of gas-discharge lamps 4, the inductive component L_resonant of the resonant circuit 5 is magnetically coupled to secondary windings 7. During the pre-heat period, inverter 3 is operated at a high switching frequency, well above the resonant frequency of the resonant circuit 5. Consequently, a relatively low AC voltage is generated by the inverter 3 and an associated pre-heat voltage is coupled from the inductive component 6 to each of the secondary windings 7.
Unfortunately, while ballast circuit 1 is very useful to operate high impedance gas-discharge lamps 4, the pre-heat voltage continues to be coupled to the lamp filaments 4A after the lamp filaments 4A have been ignited. This reduces the efficiency of ballast circuit 1 during steady state operation and may cause an over current problem. This is particularly troublesome for high impedance lamps which are particularly sensitive to over-current problems.
The prior art ballast circuit 1 utilizes capacitors C1, C2, and C3 to reduce the magnitude of the pre-heat voltage during steady state operation. However even then, the pre-heat voltage causes unacceptable inefficiencies and lamp pin current problems when operating high impedance gas-discharge lamps 4. Furthermore, the selection of capacitance values for capacitors C1, C2, and C3 requires a delicate balance between providing a pre-heat voltage at a high level during the pre-heat period to pre-heat the filaments 4A and maintaining the pre-heat voltage at a low level during steady-state operation to not cause over-current problems. This makes the sizing of capacitors C1, C2, and C3 particularly difficult and sometimes impractical.
The prior art solves this problem by coupling the prior art ballast circuit 1 to expensive filament shutdown circuits which significantly increase the cost of the ballast circuit 1.
What is needed, then, is a filament cutback circuit that will reduce the pre-heat voltage to the lamp filament after lamp ignition without significantly increasing the cost of the ballast circuit.